In the gas turbine industry, turbine blades need to be manufactured with cooling passages for the flow of cooling air therethrough. Generally, turbine blades are manufactured by an investment moulding process in which the blades are formed from a single crystal of the material.
The investment moulding process involves the use of a master mould to provide a wax precursor of the turbine blade. The wax precursor is covered with a ceramic material by being dipped in a slurry of the ceramic material which sets to form an investment mould. The investment mould is then heated to melt the wax which flows away. Finally, the molten metallic alloy material to form the turbine blade is poured into the ceramic investment mould, and the alloy material allowed to cool and solidify. The ceramic material is then dissolved using a suitable acid.
In order to form the passages in the blade, the master mould is initially provided with a ceramic core which corresponds to the shape of the cooling passage, around which the wax is poured. In order to ensure that the ceramic core is held in place when the wax is melted away, pins formed of a platinum material (called p-pins) are inserted through the wax precursor so that their ends engage the core. Generally, the pins are inserted in pairs opposite to each other to ensure that the core is held securely in place.
In order to ensure that the pins are located securely, the pins project from the wax, and a pyramidal wax formation is provided on the precursor around each p-pin.